Baxi Solo 3 60pf?

Hi can anyone shine some light on this.
Boiler on, fan comes on, pilot comes on, boiler fires. Stays on for about 30 seconds then shuts down. Will after a minute or so try again, but doesn't always get to full fire up... and you'll hear it clicking away trying to go through the cycle. Then it will try again and manage it... but again only for a minute or so.

Its not my boiler a friend of mine - and so far her 'engineers' have replaced everything but the boiler (or parts of) and relieved her of a great sum of money.... Grrr, but clearly have not solved the problem at all.

Im confused because it does manage to fire sometimes, so surely any main fault would prevent this from happening.
Any ideas greatly appreciated. Im wondering PCB fault?
Regards Rob